Osmolarity - The Definitive Guide | Dictionary of Biology (2023)


Osmolality is the total concentration of solute within a given volume of a solvent, expressed in osmoles per liter (Osm/L) or milligrams per liter (mOsm/L). These solute concentrations (solute particles) must be osmotically active—that is, they cause water to move across a selectively permeable membrane (the cell membrane) by osmosis. Osmol are measured concentrations of dissociative ions in solution that contribute to osmotic pressure.

What is osmolality?

To understand osmolarity, we need to know how water and molecules travel across cell membranes. This requires a basic knowledge of some relevant terms.


Molecular diffusion is the net movement of molecules in solid, liquid, or gaseous form from an area of ​​high molecular concentration to one of low concentration. No energy in the form of adenosine triphosphate (ATP) is required. Diffusion of the solid is its resultatomic motion under the influence of temperature. For this reason, it is rarely thought of as a passive form of molecular transport - solids need energy in the form of heat to give their atoms enough energy to disperse.

In the human body, diffusion is the passive movement of molecules in water or gas along a concentration gradient—from an area of ​​high concentration to an area of ​​low concentration. The ultimate goal of diffusion is to create the same concentration in connected areas. This can occur in the presence of a sieve-like barrier called a semipermeable membrane or without it. In human biology, this membrane is the phospholipid membrane of a cell.

Osmosis is a subcategory of diffusion, but instead of the movement of particles across a concentration gradient, it describes the movement of water molecules—always across a semipermeable membrane—from a low concentration of solute to a higher concentration of the same solute.

Semi-permeable membrane

A semipermeable membrane allows some molecules to pass through, but prevents large or charged molecules from doing the same. Osmosis always requires a semipermeable membrane. A selectively permeable membrane is not required for diffusion to occur.

The phospholipid bilayer of a cell membrane has a hydrophobic core (nonpolar) and a hydrophilic (polar) inner and outer surface. When larger molecules have an electrical charge, as in the case of ions and polar molecules, they arecannot cross the membrane. Instead, they need open channels (pores) or transport proteins to enter or leave the cell.


A solution is a mixture of solvent and solute.

(Video) Hypertonic, Hypotonic and Isotonic Solutions!

A solvent - solid, liquid or gas - can separate other molecules by changing their electrical charge and causing movement. In the body, the solvent is water. Plasma osmolality refers to the solvent of water and many dissolved substances in blood.

Water molecules are the driving force behind osmosis. During diffusion, water molecules force solutes to move across an area until the concentrations equalize. In osmosis, water moves across a membrane that is not open to solute and causes similar solute concentrations on both sides of that membrane.

The concentrations of solutes and water in blood plasma and extracellular fluid are very similar. However, the concentrations inside and outside the cell membrane can be very different. Here osmosis has its main function.


Osmosis is the movement of a solvent—not a solute—through a semipermeable membrane across a concentration gradient. In the body, it refers to the concentration of water molecules on both sides of the selectively permeable cell membrane.

The reason that water molecules are responsible for osmosis and not the solutes dissolved in them is because many solutes cannot cross the phospholipid bilayer of the cell membrane. Water molecules can.

For example, sugar molecules are too large to pass through a cell membrane and require active transport. If ten molecules of sugar dissolve in twenty molecules of water outside a cell and two molecules of sugar dissolve in one hundred molecules of water inside a cell, then the concentrations of these two solutions are not the same. Bringing more water molecules into the cell equalizes the sugar concentrations and energy is not wasted transporting the larger glucose molecules.

Isotonic solution

An isotonic solution is the goal of both osmosis and diffusion. This term describes the same concentration of dissolved substances in two areas and is called a homogeneous mixture. Cell membranes constantly allow water and solute molecules to pass from the inside of the cell to the outside and vice versa. This occurs in response to changing concentration levels of solutes. Means of transport that do not consume energy (passive transport) are the most efficient. Osmosis and diffusion are forms of passive transport.

Hypotonic solution

A hypotonic solution describes a lower concentration of a solution outside the cell than inside. If twenty molecules of glucose are dissolved in twenty molecules of water in the cytoplasm of the cell (ratio 20:20 or 1:1) and twenty molecules of glucose are dissolved in one hundred molecules of water in the extracellular fluid (ratio 20:10 or 20:10 :5), the concentration of the solution lower outside the cell.

Let us imagine that forty water molecules enter the cell by osmosis from the extracellular space. The extracellular glucose to water ratio becomes 20:60 or 1:3. Adding water inside the cell also affects the concentration level there. Twenty molecules of glucose dissolved in sixty molecules of water in the cell (20:60 or 1:3) creates an isotonic solution on both sides of the cell membrane. However, this is not always ideal. Too much water in the cell can cause it to swell. it can break and die.

(Video) 1.4 Skill: Estimation of osmolarity in tissues (Practical 2)

Hypertonic solution

In an extracellular hypertonic solution, water will let the cell reduce the high concentrations outside the membrane. The goal is to obtain an isotonic solution on both sides. In a hypertonic solution, the concentration of solutes in a solution is higher outside the cell.

The opposite effect of a hypotonic solution takes place inside the cell. He becomes dehydrated from the loss of water and may lose his ability to function and possibly die.


Molarity is a measurement that expresses the number of moles of a solute per liter of solution.

A mole is a unit of measurement calculated according to the International System of Units. It is based on the number of carbon atoms in twelve grams of carbon-12.

The International Union of Pure and Applied Chemistry has since changed the old formula. Carbon atoms have been replaced by Avogadro's mathematical constant 6.02214076 x 1023(602,214,076,000,000,000,000,000). For scientists, this change makes little difference. Avogadro's constant happens to be equal to the number of carbon atoms in twelve grams of carbon-12.

If we were just looking at the mass of an atom, each number would be small and very difficult to calculate. The definition that one mole of a substance has the same mass as twelve grams of carbon-12 makes the calculations much easier. The mole helps us convert elements and combinations of elements into a single unit of mass.

You can calculate moles by looking at the atomic weight of an element. Atomic mass appears on most periodic tables. To calculate the amount of a substance in moles, we consider the elements of that substance separately.


Osmol are units of osmolality. The calculations are not based on Avogadro's constant, but based on the different particles of a solute dissolved in water. It is expressed as Osm/kg of water.

Many solutes dissociate in water, and only solutes have osmol units that are different from mol units. When we add salt (NaCl) to water, the positive sodium ions and the negative chloride ions are separated. This gives us two different types of particles. If you have a solution of one mole of salt in water, you have an osmotic concentration made up of two particles: a sodium ion and a chloride ion. Each mole of sodium chloride is two osmoles in solution.

(Video) Osmosis and Tonicity

Calculation of osmolality

Knowing how to calculate osmolality should be much easier if you are familiar with the above information. We know that a definition of osmolarity describes the number of solute particles in one liter of solvent (water).

Example of osmolality calculation

Imagine you have 400 ml of water in which 60 g of magnesium chloride (MgCl2) is resolved.

Looking at the periodic table, write the atomic masses for magnesium (24.3) and chlorine (35.5). You can then calculate the weight in grams of one mole of magnesium chloride: 24.3 plus 35.5 plus 35.5 = 95.3 g.

To convert from grams to moles, divide the number of grams in the solution by the total atomic mass of the solute. Dividing 60 g by 95.3 g gives 0.63 moles of magnesium chloride in a 400 ml solution.

To find the number of osmoles per mole in this solution, look at the total number of particles in the magnesium chloride that has separated. As a salt, this compound dissociates into ions. One particle of magnesium and two chlorides per molecule means that each mole of MgCl2it is three osmolles.

To calculate how many osmoles are in your 400 ml solution, multiply the moles (0.63) by the osmoles (3). There are 1.89 osmoles of solute in 400 ml of water.

However, the result of an osmolarity formula is expressed in osmoles per liter. Now you need to divide 1.89 osmol by 0.4 liters (400 ml) to arrive at the answer - the osmolarity of this solution is 4.73 Osm/L.

If you know the osmolarity of two solutions on opposite sides of a semipermeable membrane, you can calculate the direction of osmosis. Water moves to the side with higher osmolality. It moves from the thinner side to the more concentrated side. The side with high osmolarity is hyperosmotic to the other (hypoosmotic) side. If the concentrations are the same, they are isosmotic.

Osmolality vs osmolarity

Osmolality is another measurement of dissolved osmolality, but in a kilogram of solvent instead of a liter of solvent (osmolality). While solutes are measured by weight or mass, solvents are measured by weight (kilograms) or volume (liters). Osmolality is expressed in Osm/L and osmolality in Osm/kg. Because water can change in volume with temperature, many scientists prefer osmolarity. A kilo remains a kilo regardless of temperature; However, a liter of water and a liter of ice do not occupy the same space.

(Video) Transport in Cells: Diffusion and Osmosis | Cells | Biology | FuseSchool

Blood plasma osmolarity is not a test - plasma osmolarity is. That countsconcentration of dissolved substances in the blood according to the weight of the blood. Results are given in milliosmoles per kilogram (mOsm/kg).

Osmosity vs. tonicity

How permeable a cell membrane is and the solute concentrations on either side affect the tonicity of an extracellular solution. How much pressure a cell membrane can withstand and how well it pumps water also affects how much extracellular fluid enters or leaves the cell. We have already looked at hypotonic, isotonic and hypertonic solutions - these are levels of solution tonicity. Osmolarity depends mainly on the tonicity of the liquid environment, but tonicity does not depend on osmosis.

Osmolarity vs molarity

Osmolality is the number of dissolved osmoles in one liter of solution. Molarity is the number of moles of solute in one liter of solution. Because many substances break down in water, they are often easier to calculate by osmolality. However, if a solution does not contain dissociated molecules, such as glucose in water, each mole of solute is also an osmole.

The difference between osmolarity and molarity is explained byvan't Hoff-factor- the number of moles (not mass or weight) of solute particles (ions) in a solute.

For example, a 1 mol/l glucose solution does not dissociate. the van't Hoff factor is therefore one. A solution of 1 mol/L glucose (molarity) has an osmolarity of 1 Osm/L.

However, 1 mol/L calcium chloride solution (CaCl2) dissociates into three ions. It contains one mole of calcium ions and two moles of chloride ions. 1 mol/L solution (molarity) multiplied by a van't Hoff factor of three individual particles means 1 mol CaCl2is 3 Osm/L.


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(Video) Serum Osmolality (What is the Osmolarity of your plasma?)


Osmolarity - The Definitive Guide | Dictionary of Biology? ›

Definition. Osmolarity is the total solute concentration within a specific volume of a solvent expressed in osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L).

What is osmolarity in biology? ›

The term osmolarity refers to the number of particles of solute per liter of solution, whereas the term osmolality refers to the number of particles of solute per kilogram of solvent.

What is osmolarity in biology IB? ›

Osmolarity is a measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L) Solutions may be loosely categorised as hypertonic, hypotonic or isotonic according to their relative osmolarity.

What is osmolarity and its importance? ›

Osmolality indicates the concentration of all the particles dissolved in body fluid. It is routinely measured in clinical laboratories for the differential diagnosis of disorders related to hydrolytic balance regulation, renal function, and small-molecule poisonings.

What is osmolarity simply? ›

Simply put, osmolality is a measurement of the total number of solutes in a liquid solution expressed in osmoles of solute particles per kilogram of solvent.

What are the concepts of osmolarity? ›

Osmole is unit of the amount of substance, one mole of nonionized impermeant solute is one osmole. Assuming an ideal solution, osmotic pressure (π) in mmHg is 19.3 times the osmolarity. Osmolarity is defined as the number of milliosmoles of the solutes per liter of solution.

What is the easiest way to calculate osmolarity? ›

  2. The osmolarity of solutions containing a single type of solute (for example: just glucose or just sodium chloride) can be calculated from the following equation:
  3. osmolarity = molarity x n x f.

What is osmolarity and how is it measured? ›

What is osmolarity? Osmolarity is an estimation of the osmolar concentration of plasma and is proportional to the number of particles per litre of solution; it is expressed as mmol/L. This is what is used when a calculated value is derived. It is derived from the measured Na+, K+, urea and glucose concentrations.

What are the three types of osmolarity? ›

Three terms—hyperosmotic, hypoosmotic, and isoosmotic—are used to describe relative osmolarities between solutions.

What are the factors affecting osmolarity? ›

Osmolarity is affected by changes in water content, as well as temperature and pressure. In contrast, osmolality is independent of temperature and pressure.

What does the osmolarity depend on? ›

…its overall osmotic concentration, or osmolality. This depends on the concentration of solutes. While all solutes contribute to osmolality, small particles such as sodium or chloride ions are influential out of all proportion to their weight, and indeed account for over 90 percent of the osmolality of plasma.

What happens when osmolarity is high? ›

In healthy people, when osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH). This hormone causes the kidneys to reabsorb water. This results in more concentrated urine. The reabsorbed water dilutes the blood.

What is an example of osmolarity? ›

osmolarity. Osmolarity is dependent upon the number of impermeant molecules in a solution, not on the identity of the molecules. For example, a 1M solution of a nonionizing substance such as glucose is a 1 Osmolar solution; a 1M solution of NaCl = 2 Osm; and a 1M solution of Na2SO4 =3 Osm.

What is the osmolarity of the human body? ›

Osmolarity is defined as the number of particles per liter of fluid. Physiologic blood plasma osmolarity is approximately 286 mOsmoles/L. Less than this is hypoosmotic, and greater is hyperosmotic.

Why is osmolarity important for humans? ›

When osmolality increases, it triggers your body to make antidiuretic hormone (ADH). It's also called arginine vasopressin (AVP). This hormone tells your kidneys to keep more water inside your blood vessels and your urine becomes more concentrated. When osmolality decreases, your body doesn't make as much ADH.

Why is osmolarity important for biological solutions? ›

1 Answer. Osmolarity matters because cells cannot survive if the osmolarity if their surroundings is much different from their own. Water moves across a membrane from a lower osmolarity to a higher osmolarity. In other words, it moves from the dilute side to the concentrated side.

How does osmolality affect cells? ›

Osmolality has been linked to both cell volume regulation and recombinant protein productivity. Typically, hyperosmotic conditions lead to an increase in cell size [22]. There have been several studies in which, for example, hyperosmolar conditions were introduced to increase recombinant protein titre [23,24].

What is the most common method used to measure the osmolality? ›

The osmometer is a device used in clinical laboratories for measuring the concentration of particles in a solution, known as the osmolar concentration. This quantity can be expressed as osmolality (in units of mmol/kg) or osmolarity (in units of mmol/L). Clinical laboratories usually measure osmolality.

What is the osmolarity of water? ›

There are 1.89 osmoles of solute in 400 ml of water. However, an osmolarity formula result is expressed in osmoles per liter.

Which hormone most affects the osmolarity of blood? ›

ADH is the primary hormone responsible for tonicity homeostasis. Hyperosmolar states most strongly trigger its release. ADH is stored in neurons within the hypothalamus. These neurons express osmoreceptors that are exquisitely responsive to blood osmolarity and respond to changes as little as two mOsm/L.

What is the principle of osmolality? ›

The urine osmolality determination is based upon the principle that increased concentration of a solute in a solution causes lowering of its freezing point. This method is referred to as freezing point depression osmometry.

What is osmolarity a measure of the total? ›

Osmolality measures the total number of dissolved particles per unit of solution (in mmol/L or mOsm/kg water) and reflects the concentrating ability of the kidney better than specific gravity.

What is the best formula for calculated osmolality? ›

Urine osmolality can be predicted accurately and precisely using urine urea, sodium and glucose with the following equation: Uosm = 1.25 × urea (mmol/l) or 20.87 × urea (g/l) + 1.1 × sodium (mmol/l) + 67 × glucose (mmol/l) or 3.72 × glucose (mg/dl).

What is osmolarity with equation? ›

(Osmolarity) OSM = M x the number of particles of dissociation. * For example: Like Sodium chloride(NaCl) in water dissociates into two particles (Na+ and Cl) and Magnesium chloride(MgCl2) in water dissociates into 3 particles (Mg++ and 2 Cl).

Is osmosis and osmolarity the same? ›

For osmosis, the measure of concentration changes from molarity to osmolarity. Osmolarity is defined by osmoles of solute per liter of solution e.g. 1 mol/L solution of KCl has molarity (M) of 1 but its osmolarity (OsM) is 2 because KCl dissociates into K+ and Cl- in water.

What is the difference between high and low osmolarity? ›

Higher osmolality means you have more particles in your serum. Lower osmolality means the particles are more diluted. Your blood is a little like a liquid chemistry set. Along with oxygen, it contains proteins, minerals, hormones, and a long list of chemicals.

Is osmolarity the same as blood pressure? ›

When there is a large intake of $N{{a}^{+}}$ in less amount of water, our body reacts through osmoreceptors and detects high osmolarity that result in a higher amount of pressure of blood flow in the arteries. Hence, when due to less amount of water, the osmolarity becomes high, the blood pressure increases.

Does drinking water affect osmolarity? ›

Individuals with low water intake have higher plasma VP and copeptin concentration, and as a result, higher urine osmolality (u-Osm) than individuals with higher water intake [1, 18], whereas increased water intake effectively lowers circulating VP, copeptin, and u-Osm [1, 19].

What determines the osmolarity of a fluid? ›

Osmolality, estimated (serum)

Osmolality of extracellular fluid (ECF) is determined predominantly by electrolytes, especially sodium, and small molecules (glucose and urea) and is reflective of fluid shifts between the vascular space and the interstitium.

What regulates osmolarity? ›

In humans, the osmolarity of blood plasma in the body is normally kept around 290 to 300 mOsm/L and is tightly controlled by antidiuretic hormone (ADH). With ADH secretion, the kidney increases the reabsorption of water back into circulation and causes vasoconstriction to bring up blood pressure.

What two factors regulate body fluid osmolarity? ›

Regulation of osmolarity is achieved by balancing the intake and excretion of sodium with that of water. (Sodium is by far the major solute in extracellular fluids, so it effectively determines the osmolarity of extracellular fluids.)

Which factor helps in maintaining an increasing osmolarity? ›

The proximity between the Henle's loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium.

What is osmolarity in osmosis? ›

Osmolarity is a measure of the concentration of osmotically active particles in a solution. It is sometimes called a “colligative” property of the solution by chemists because it depends on the number of particles in a volume of solution rather than the identity of the particles.

Does high osmolarity mean more water? ›

When there is less water in your blood, the concentration of particles is greater. Osmolality increases when you are dehydrated and decreases when you have too much fluid in your blood.

What is osmolarity in the urinary system? ›

An osmolality test measures the concentration of particles in a solution. In this case, the solution is urine. An osmolality urine test is performed to measure the concentration of particles in urine. Greater than normal results may indicate conditions such as Addison disease, congestive heart failure or shock.

What does osmolarity mean in kidney? ›

Osmolality is a measure of urine concentration which is more objective than self-reported fluid intake. It has a positive association with hypovolemia. However, it remains controversial whether osmolality is associated with decreased kidney function and/or albuminuria.

How does osmolarity affect cells? ›

Changes in extracellular osmolarity alter cell volume, and therefore, the concentration of intracellular macromolecules. In turn, intracellular macromolecule concentration is a key physical parameter affecting the spatial organization and pressurization of the nucleus.

What is osmolarity and how does it affect water movement? ›

Osmolarity is a measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane (one which allows free passage of water and completely prevents movement of solute) compared to pure water.

What happens when osmolarity increases? ›

In healthy people, when osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH). This hormone causes the kidneys to reabsorb water. This results in more concentrated urine. The reabsorbed water dilutes the blood.

What happens if osmolarity is too high? ›

In healthy people, when osmolality in the blood becomes high, the body releases antidiuretic hormone (ADH). This hormone causes the kidneys to reabsorb water. This results in more concentrated urine. The reabsorbed water dilutes the blood.

What happens to osmolality when you drink water? ›

Individuals with low water intake have higher plasma VP and copeptin concentration, and as a result, higher urine osmolality (u-Osm) than individuals with higher water intake [1, 18], whereas increased water intake effectively lowers circulating VP, copeptin, and u-Osm [1, 19].

What happens when osmolarity decreases? ›

Lowered osmolarity decreases ADH secretion, causing loss of water over salt in the kidney and the blood osmolarity returns toward normal. Increased osmolarity increases ADH secretion, leading to reabsorption of water. Salt can be excreted in excess of water, leading to a return toward normal plasma osmolarity.

What is the osmolarity of body fluid? ›

Osmolarity is defined as the number of particles per liter of fluid. Physiologic blood plasma osmolarity is approximately 286 mOsmoles/L. Less than this is hypoosmotic, and greater is hyperosmotic.

What does increased urine osmolarity mean? ›

Increased urine output and a high osmolality may occur due to your body flushing out a substance, such as excess glucose if you have diabetes. Your doctor will work with you to figure out what's causing your abnormal results.

Does high osmolarity mean higher blood pressure? ›

Hence, when due to less amount of water, the osmolarity becomes high, the blood pressure increases.

Does high osmolarity mean low water potential? ›

What is the relationship between osmolarity and water potential energy? As osmolarity of a solution (i.e. concentration of solute particles) increases, the water potential energy decreases. WHY? Water molecules move from regions of high water potential energy to regions of lower water potential energy.


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